It is commonplace today to apply marks, such as one- or two dimensional bar codes, to objects, such as consumer products, food goods, beverage packs, cans and bottles, cigarette packages and other tobacco products, documents, certificates, money bills and the like. Marks can then serve the purpose of tracking, identifying, or authenticating the objects in the field, i.e. in the market, in a production or packaging line, at a retailer's place, during shipping, and the like.
Once a mark is applied to an object, the encoded information can then be later retrieved by means of mark (barcode) reading devices. Such devices usually first obtain said image data that was acquired using, for example, a digital camera. Other acquisition support may be provided by means of illumination devices, such as LEDs, lasers and other light sources. The reading devices may then employ processing resources, e.g. in the form of a microprocessor (CPU) and an associated memory, for processing the obtained image data. Usually, such processing involves isolating (identifying) the barcode in the image data and decoding the payload data. The decoded data can then be further processed, displayed, or transmitted to other entities.
Marks as such appear in various fashions, of which two examples are shown in FIGS. 1A and 1B: The common one-dimensional bar code 10′ of FIG. 1A usually comprises an arrangement of elements as, for example, black and while lines 1′, 2′. Information is encoded by concatenating pre-defined groups of black and white lines 1′, 2′ of varying thickness and distance. These groups are usually associated to a specific character or meaning by some kind of industry standard.
FIG. 1B shows a common two-dimensional bar code 10″ that encodes information by means of arranging, in general terms, first type elements 1″ and second type elements 2″, such as rectangles, dots, triangles and the like, along two dimensions in some sort of ordered grid. The example of FIG. 1B follows an implementation according to the GS1 (Trademark) DataMatrix ECC 200 standard (GS1 being an international association providing standards for two-dimensional barcodes). This standard, for example, employs a so-called “L finder pattern” 4 (also called L-shape solid line, L-line, solid line, etc.) and a so-called “clock track” 3 (also called clock line, L-shape clock line etc.) surrounding the data 5 that carries the actual payload data of the bar code.
In both cases of one-dimensional and two-dimensional bar codes, at least two distinguishable types of elements are used. For example, a white-printed square as a first type element may represent the information 0, whereas a black-printed square as a second type element represents the information 1. In any way, however, implementations by means of black and white lines or dots (elements) represent just one example.
Specifically, the bar codes can be well implemented also by using color and/or fluorescent dyes or inks, thermo printing on heat-sensitive paper, mechanical means, such as milling, embossing, grinding, or physical/chemical means, such as laser etching, acid etching, etc. Any type of implementation is possible as long as the elements can be distinguished into their respective type in, for example, image data that has been obtained from the two-dimensional bar code being normally applied to some kind of object (good). For example, a digital camera can obtain digital image data of the bar code that is printed on a paper document or laser-etched on a metal can.
As such, luminescent materials as such are commonly used in security marks to be disposed on documents or articles (object), or in the bulk material of documents or articles, as an authenticity feature. A luminescent material typically converts energy of an exciting radiation of a given wavelength into emitted light having another wavelength. Luminescence emission used for authentication of a mark can lie in the spectrum range from ultraviolet (UV) light (below 400 nm), visible light (400-700 nm) or near to mid infrared light (NIR, MIR, IR) (700-2500 nm). In this context, a so-called “up-converter” material emits radiation at a shorter wavelength than the exciting radiation. By contrast, a “down-converter” material emits radiation at a longer wavelength than the exciting radiation. Most luminescent materials can be excited at more than one wavelength, and some luminescent materials can emit simultaneously at more than one wavelength.
Luminescence may be divided in the so-called “phosphorescence”, which relates to time-delayed radiation emission observable after the excitation radiation is removed (typically, with a decay lifetime from above about 1 μs to about 100 s), and so-called “fluorescence”, which relates to prompt radiation emission upon excitation (typically, with a decay lifetime below 1 μs).
Thus, a luminescent material used for a mark, upon illumination with excitation light within an excitation wavelength range, typically emits luminescence light within an emission wavelength range, which may differ from or overlap with said excitation wavelength range (depending on the material used). The characteristic spectral properties of a luminescent material such as its emission light intensity profile with time, or its characteristic decay time after excitation has stopped, for example, can be employed as a signature of this material and may thus be further used as an security feature for detecting genuineness or forgery (authenticity).
Luminescent materials can thus be ingredients of security inks or coatings. For example, the following patents disclose luminescent substances (which may include mixtures of pigments having distinct decay time properties) and security paper including such substances: EP 0 066 854 B1, U.S. Pat. Nos. 4,451,530, 4,452,843, 4,451,521. Processes and apparatuses for detecting luminescence light and authenticity of a marked item are also well known: see, for example, U.S. Pat. Nos. 4,598,205, or 4,533,244 (which disclose sensing decay behavior of luminescence emissions). Luminescent coded symbols are known from U.S. Pat. No. 3,473,027, and an optical reader for luminescent codes is disclosed in U.S. Pat. No. 3,663,813. The U.S. Pat. Nos. 6,996,252 B2, 7,213,757 B2 and 7,427,030 B2 disclose using two luminescent materials, having distinct decay time properties, for authenticating an item.
The great variety of possible implementations results also in widely varying optical properties of the mark. For example, barcodes can be printed using special inks, such as fluorescent or phosphorescent inks that emit light at different wavelengths (as compared to the wavelengths used for illumination) and/or with a delay. These specific characteristics of particular inks can be employed for authenticating a mark.
However, being able to detect specific characteristics of marks also requires proper illumination, so that the appropriate illumination wavelengths are available to which some mark responds. Usually, a high-intensity broadband light source is employed so as to ensure that sufficient intensity is provided in all wavelengths under consideration. Such prerogatives pose high demands on the corresponding light sources used for illuminating a mark, in that the emission power characteristics of a given light source are exploited to some maximum extent.
Such operation, however, may result in an increased or even impermissible generation of heat, so that additional means for cooling the light source may become necessary. Furthermore, light source operation close or even above the maximum power ratings can dramatically reduce the life-time of the involved components. Once the light-source becomes too hot or has even degraded, also the corresponding reading device will fail, since proper illumination is no longer possible.
At the same time, it is nowadays common to use handheld or even wireless reading devices that only feature power sources of limited capacity (battery). In this way, also a short-term effect can be observed in that excessive power consumption by the light source results in an increased downtime of the device, during which batteries have to be replaced or charged, and, in turn, the device cannot be used. Further, any additional measures for cooling a light source in handheld devices are clearly not desirable, since they add to the device weight, size, and—again—power consumption.
Conventional light sources for such readers include incandescent lamps (typically for wavelengths between about 400 nm to about 2500 nm), flash lamps (like Xenon high-pressure flash lamp, for example), laser or Light-Emitting-Diodes (LEDs, emitting in the UV, visible or IR regions, typically for wavelengths from about 250 nm to about 1 micron). Conventional light sources are powered via a drive current (a LED for example) or via drive voltage (discharge lamps, for example). As an example, composite light sources with multi-LED modules (equipped with collimating and mixing structure) are disclosed in the US patent application US 2009/0316393 A1 (see also the U.S. Pat. No. 7,125,143 B2 and the European patent EP 1 815 534 B1).
In other words, the light source should deliver illumination to a mark so that the emission light intensity is sufficient for measurement operations. Due to the fact that only a part of the illumination light corresponds to a sub-bandwidth that is actually used for excitation, a problem of heat dissipation can arise for the light source. This may require controlling the heat within the light source to avoid damages to the source and/or lifecycle decrease. Such techniques include, for example, a specific design of the LEDs themselves and/or their arrangement on adapted substrates, and also cooling systems.
Therefore, there is a need for an improved mark reader devices that avoids overheating of the light source, maximizes the life span of the light source and the mark device as a whole, and reduces the overall size, weight, and power consumption.